Continuous dimming of incandescent lamps is typically accomplished using triac-equipped wall or table-top dimmer circuits connected in line with the lamp bulb, using so-called “phase cut” dimming where a portion of the line AC waveform is essentially removed in each AC cycle to reduce the light output. Integral electronic lamps such as compact fluorescent designs (CFLs) and LED lamps have recently become more popular in which the lamp includes electronic driver circuitry to generate high frequency AC power to drive a fluorescent light source or DC to drive one or more LEDs. These devices can be used in conventional lamps designed for incandescent bulbs and may include dimming circuitry allowing the light output to be adjusted by phase-cut dimming (triac control). Phase cut wall dimming circuits use triac that require a minimum amplitude of latching current and holding current to remain conductive until the completion of the line power half-cycle.
Conventional, capacitive input filter rectifier circuits for electronic lighting power circuits cannot provide these requirements, particularly for low dimming levels, where input capacitance can disrupt the continuity of the triac and its firing circuit. Low power resistive inputs may also not be enough to meet the triac requirements. Aperiodic triac operation can produce anon periodic variation of the input power, leading to flicker in lighting systems. Other types of boost circuits have been used which draw a sinusoidal input current, but these can cause the dimmer triac to stop conducting before the end of the power line half-cycle. Because the input current is not constant in these circuits, the triac current may drop below the triac holding current and triac conduction cannot be maintained. Other passive techniques have also been employed which cannot maintain a constant input current. It is therefore desirable to provide a constant current load in lighting system power circuits to prevent early turn off of triac type dimming devices.